Allometry Of Sexual Size Dimorphism Research Articles

Sexual size dimorphism and sex ratio in arthropod ectoparasites: contrasting patterns at different hierarchical scales

The aims of this study were to determine whether sexual size dimorphism in fleas and gamasid mites (i) conforms to Rensch’s rule (allometry of sexual size dimorphism) and (ii) covaries with sex ratio in infrapopulations (conspecific parasites harboured by an individual host), xenopopulations (conspecific parasites harboured by a population of a given host species in a locality) and suprapopulations (conspecific parasites harboured by an entire host community in a locality). Rensch’s rule in sexual size dimorphism was tested across 150 flea and 55 mite species, whereas covariation between sexual size dimorphism and sex ratio was studied using data on ectoparasites collected from small mammalian hosts in Slovakia and western Siberia. For fleas, we controlled for the confounding effect of phylogeny. The slope of the linear regression of female size on male size was significantly smaller than 1 in fleas, but did not differ from 1 in mites. The proportion of males in flea infrapopulations significantly increased with an increase in the female-to-male body size ratio. The same was true for obligatory haematophagous mites. No relationship between sex ratio and sexual size dimorphism was found for xenopopulations of either taxon or for mite suprapopulations. However, when controlling for the confounding effect of phylogeny, a significant negative correlation between sex ratio and sexual size dimorphism was revealed for flea suprapopulations. We conclude that (i) some macroecological patterns differ between ectoparasite taxa exploiting the same hosts (allometry in sexual size dimorphism), whereas other patterns are similar (sexual size dimorphism-sex ratio relationship in infrapopulations), and (ii) some patterns are scale-dependent and may demonstrate the opposite trends in parasite populations at different hierarchical levels.

Rensching cats and dogs: feeding ecology and fecundity trends explain variation in the allometry of sexual size dimorphism.

The tendency for sexual size dimorphism (SSD) to increase with body mass in taxa where males are larger, and to decrease when females are larger, is known as Rensch's rule. In mammals, where the trend occurs, it is believed to be the result of a competitive advantage for larger males, while female mass is constrained by the energetics of reproduction. Here, we examine the allometry of SSD within the Felidae and Canidae, demonstrating distinctly different patterns: in felids, there is positive allometric scaling, while there is no trend in canids. We hypothesize that feeding ecology, via its effect on female spacing patterns, is responsible for the difference; larger male mass may be advantageous only where females are dispersed such that males can defend access to them. This is supported by the observation that felids are predominately solitary, and all are obligate carnivores. Similarly, carnivorous canids are more sexually dimorphic than insectivores and omnivores, but carnivory does not contribute to a Rensch effect as dietary variation occurs across the mass spectrum. The observed inter-familial differences are also consistent with reduced constraints on female mass in the canids, where litter size increases with body mass, versus no observable allometry in the felids.

Allometry of sexual size dimorphism in turtles: a comparison of mass and length data.

BackgroundThe macroevolutionary pattern of Rensch’s Rule (positive allometry of sexual size dimorphism) has had mixed support in turtles. Using the largest carapace length dataset and only large-scale body mass dataset assembled for this group, we determine (a) whether turtles conform to Rensch’s Rule at the order, suborder, and family levels, and (b) whether inferences regarding allometry of sexual size dimorphism differ based on choice of body size metric used for analyses.MethodsWe compiled databases of mean body mass and carapace length for males and females for as many populations and species of turtles as possible. We then determined scaling relationships between males and females for average body mass and straight carapace length using traditional and phylogenetic comparative methods. We also used regression analyses to evalutate sex-specific differences in the variance explained by carapace length on body mass.ResultsUsing traditional (non-phylogenetic) analyses, body mass supports Rensch’s Rule, whereas straight carapace length supports isometry. Using phylogenetic independent contrasts, both body mass and straight carapace length support Rensch’s Rule with strong congruence between metrics. At the family level, support for Rensch’s Rule is more frequent when mass is used and in phylogenetic comparative analyses. Turtles do not differ in slopes of sex-specific mass-to-length regressions and more variance in body size within each sex is explained by mass than by carapace length.DiscussionTurtles display Rensch’s Rule overall and within families of Cryptodires, but not within Pleurodire families. Mass and length are strongly congruent with respect to Rensch’s Rule across turtles, and discrepancies are observed mostly at the family level (the level where Rensch’s Rule is most often evaluated). At macroevolutionary scales, the purported advantages of length measurements over weight are not supported in turtles.

Variable patterns of intraspecific sexual size dimorphism and allometry in three species of eusocial corbiculate bees

Sexual size dimorphism (SSD), in which one sex is larger than the other, has remained understudied in social insects, particularly bees. Using weight and linear structural measurements, we quantified the magnitude of SSD and its variation across nests in three species of corbiculate bees, two belonging to the highly eusocial Apini (Apis mellifera) and Meliponini (Melipona beecheii), and one to the primitively eusocial Euglossini (Euglossa viridissima). We asked if similar to most insects, including Hymenoptera, SSD is female-biased in these eusocial species. Contrary to expectations, we found that SSD was moderately male-biased in the two highly eusocial species and slightly male-biased for weight and not significant for linear size in E. viridissima. The possible roles of queen protogyny and reduced brood provisioning by queens in shaping these patterns of SSD are discussed. The allometry of SSD among nests differed among species as well, ranging from hypoallometry in A. mellifera, to isometry in M. beecheii, to hyperallometry in E. viridissima. This variation indicates that the phenotypic response of body size to differing conditions across nests differs both between sexes and among species. The variation detected among the three studied species in both SSD and allometry for SSD precludes any broad generalizations to other corbiculate bees. However, it does suggest that corbiculate bees can provide a new and diverse framework to analyze the effects of social environment on the evolution of animal sexual dimorphism.

Lack of support for Rensch's rule in an intraspecific test using red flour beetle (Tribolium castaneum) populations

Rensch's rule proposes a universal allometric scaling phenomenon across species where sexual size dimorphism (SSD) has evolved: in taxa with male-biased dimorphism, degree of SSD should increase with overall body size, and in taxa with female-biased dimorphism, degree of SSD should decrease with increasing average body size. Rensch's rule appears to hold widely across taxa where SSD is male-biased, but not consistently when SSD is female-biased. Furthermore, studies addressing this question within species are rare, so it remains unclear whether this rule applies at the intraspecific level. We assess body size and SSD within Tribolium castaneum (Herbst), a species where females are larger than males, using 21 populations derived from separate locations across the world, and maintained in isolated laboratory culture for at least 20 years. Body size, and hence SSD patterns, are highly susceptible to variations in temperature, diet quality and other environmental factors. Crucially, here we nullify interference of such confounds as all populations were maintained under identical conditions (similar densities, standard diet and exposed to identical temperature, relative humidity and photoperiod). We measured thirty beetles of each sex for all populations, and found body size variation across populations, and (as expected) female-biased SSD in all populations. We test whether Rensch's rule holds for our populations, but find isometry, i.e. no allometry for SSD. Our results thus show that Rensch's rule does not hold across populations within this species. Our intraspecific test matches previous interspecific studies showing that Rensch's rule fails in species with female-biased SSD.

Allometry of sexual size dimorphism in domestic dog.

BackgroundThe tendency for male-larger sexual size dimorphism (SSD) to scale with body size – a pattern termed Rensch's rule – has been empirically supported in many animal lineages. Nevertheless, its theoretical elucidation is a subject of debate. Here, we exploited the extreme morphological variability of domestic dog (Canis familiaris) to gain insights into evolutionary causes of this rule.Methodology/Principal FindingsWe studied SSD and its allometry among 74 breeds ranging in height from less than 19 cm in Chihuahua to about 84 cm in Irish wolfhound. In total, the dataset included 6,221 individuals. We demonstrate that most dog breeds are male-larger, and SSD in large breeds is comparable to SSD of their wolf ancestor. Among breeds, SSD becomes smaller with decreasing body size. The smallest breeds are nearly monomorphic.Conclusions/SignificanceSSD among dog breeds follows the pattern consistent with Rensch's rule. The variability of body size and corresponding changes in SSD among breeds of a domestic animal shaped by artificial selection can help to better understand processes leading to emergence of Rensch's rule.

Allometry of Sexual Size Dimorphism in Dioecious Plants: Do Plants Obey Rensch’s Rule?

Rensch's rule refers to a pattern in sexual size dimorphism (SSD) in which SSD decreases with body size when females are the larger sex and increases with body size when males are the larger sex. Many animal taxa conform to Rensch's rule, but it has yet to be investigated in plants. Using herbarium collections from New Zealand, we characterized the size of leaves and stems of 297 individuals from 38 dioecious plant species belonging to three distantly related phylogenetic lineages. Statistical comparisons of leaf sizes between males and females showed evidence for Rensch's rule in two of the three lineages, indicating SSD decreases with leaf size when females produce larger leaves and increases with leaf size when males produce larger leaves. A similar pattern in SSD was observed for stem sizes. However, in this instance, females of small-stemmed species produced much larger stems than did males, but as stem sizes increased, SSD often disappeared. We hypothesize that sexual dimorphism in stem sizes results from selection for larger stems in females, which must provide mechanical support for seeds, fruits, and dispersal vectors, and that scaling relationships in leaf sizes result from correlated evolution with stem sizes. The overall results suggest that selection for larger female stem sizes to support the weight of offspring can give rise to Rensch's rule in dioecious plants.

Why are reproductively parasitic fish males so small?—influence of tactic-specific selection

Despite the wide prevalence of alternative reproductive tactics, little attention has been paid to why reproductively parasitic males are so small. In this study, we tackled this issue in a shell-brooding fish Lamprologus callipterus. Sneaky 'dwarf males' of this fish remain much smaller than bourgeois conspecifics throughout their life and employ a unique parasitic tactic, i.e. entering into a gastropod shell where a female is spawning, passing through the space between the female and shell wall and staying behind her to ejaculate throughout the spawning event. Here, we tested the prediction that they remain small to get past her through the shell spaces by interpopulation comparison. We showed, across populations, a negative allometry for sexual size dimorphism, an exponential increase of female size with an increase in shell size and a negative correlation between the magnitude of sexual size dimorphism and shell size. These results suggest that the inner spaces strongly regulate dwarf male size. We conclude that the small bodies of dwarf males arise from adaptation to their unique reproductive behaviour.

Unusual allometry for sexual size dimorphism in a cichlid where males are extremely larger than females

When males are the larger sex, a positive allometric relationship between male and female sizes is often found across populations of a single species (i.e. Rensch's rule). This pattern is typically explained by a sexual selection pressure on males. Here, we report that the allometric relationship was negative across populations of a shell-brooding cichlid fish Lamprologus callipterus, although males are extremely larger than females. Male L. callipterus collect and defend empty snail shells in each of which a female breeds. We found that, across six populations, male and female sizes are positively correlated with not only sexual and fecundity selection indices, but also with shell sizes. Given their different reproductive behaviours, these correlations mean that males are required to be more powerful, and thus larger, to transport larger shells, while female bodies are reduced to the shell size to enable them to enter the shells. Among the three size selections (sexual selection, fecundity selection and shell size), shell size explained the allometry, suggesting that females are more strongly subject to size selection associated with shell size availability than males. However, the allometry was violated when considering an additional population where size-selection regimes of males differed from that of other populations. Therefore, sexual size allometry will be violated by body size divergence induced by multiple selection regimes.

An Assessment of Allometry for Sexual Size Dimorphism in Mainland Anoles

ABSTRACT. Allometry for sexual size dimorphism (SSD) occurs when the degree of SSD varies with overall body size and is common in animals, including lizards. Previous reviews of allometry for SSD showed ‘Iguanid’ lizards to be comprised of mostly malelarger species which showed weak and statistically non-significant allometry for SSD. Beta-anoles (or Norops) are better suited for analysis, however, because they show extensive, up-to-date phylogenetic support, and presumably share certain ecological and life history traits. Here, I use phylogenetically independent contrasts of log male and log female size to assess allometry for SSD in 23 species of mainland anoles. The species of Norops measured here showed mixed-bias SSD (i.e., both male-larger and female-larger species) and the degree of SSD varied considerably. The slope of MA regression (and its 95% confidence interval) indicated that Norops showed isometry and did not exhibit allometry consistent with Rensch's Rule. Moreover, a pairwise t-test of ind...

Sexual selection explains sex-specific growth plasticity and positive allometry for sexual size dimorphism in a reef fish

In 1950, Rensch noted that in clades where males are the larger sex, sexual size dimorphism (SSD) tends to be more pronounced in larger species. This fundamental allometric relationship is now known as 'Rensch's rule'. While most researchers attribute Rensch's rule to sexual selection for male size, experimental evidence is lacking. Here, we suggest that ultimate hypotheses for Rensch's rule should also apply to groups of individuals and that individual trait plasticity can be used to test those hypotheses experimentally. Specifically, we show that in the sex-changing fish Parapercis cylindrica, larger males have larger harems with larger females, and that SSD increases with harem size. Thus, sexual selection for male body size is the ultimate cause of sexual size allometry. In addition, we experimentally illustrate a positive relationship between polygyny potential and individual growth rate during sex change from female to male. Thus, sexual selection is the ultimate cause of variation in growth rate, and variation in growth rate is the proximate cause of sexual size allometry. Taken together, our results provide compelling evidence in support of the sexual selection hypothesis for Rensch's rule and highlight the potential importance of individual growth modification in the shaping of morphological patterns in Nature.

The evolution of sexual size dimorphism in cottontail rabbits (Sylvilagus, Leporidae)

In mammals, ‘female-biased’ sexual size dimorphism (SSD), in which females are larger than males, is uncommon. In the present study, we examined Sylvilagus, a purported case of female-biased SSD, for evolutionary correlations among species between SSD, body-size, and life-history variables. We find that: (1) although most species are female-biased, the degree and direction of SSD vary more than was previously recognized and (2) the degree of SSD decreases with increasing body size. Hence, Sylvilagus provides a new example, unusual for a female-biased taxon, in which allometry for SSD is consistent with ‘Rensch's Rule’. As a corollary to Rensch's Rule, we observe that changes in SSD in Sylvilagus are typically associated with larger, more significant changes in males than females. Female-biased SSD could be produced by selection for larger females, smaller males, or both. Although larger female size may be related to high fecundity and the extremely rapid fetal and neonatal growth in Sylvilagus, we find little evidence for a correlation between SSD and various fecundity-related traits in among-species comparisons. Smaller male size may confer greater reproductive success through greater mobility and reduced energetic requirements. We propose that a suite of traits (female dispersion, large male home ranges, reduced aggression, and a promiscuous mating system) has favoured smaller males and thus influenced the evolution of SSD in cottontails.

Sexual Size Dimorphism and Ecomorphology of Spotfin Shiner (Cyprinella spiloptera) from the Wabash River Watershed

ABSTRACT We quantified body morphology in individuals from 49 collections of spotfin shiner (Cyprinella spiloptera) in the Wabash River watershed. Ten morphological measurements were used to produce principal component (PC) axes that were examined for relationships with watershed area. Analyses of males and females resulted in sexual dimorphism for one PC axis that represented caudal fin length and head depth; both have increased size in males. An analysis of body shape with watershed area resulted in different relationships for males and females. Caudal peduncle length increased with stream size for males and decreased with stream size for females. No significant relationships were found for sexual size dimorphism (SSD) with stream size. Allometry for SSD supported Rensch's rule—male body size increased more rapidly than female body size with increased SSD. Morphology in spotfin shiners is constrained by a combination of sexual selection and the hydrologic regime of the watershed.

Sexual selection explains Rensch's rule of allometry for sexual size dimorphism

In 1950, Rensch first described that in groups of related species, sexual size dimorphism is more pronounced in larger species. This widespread and fundamental allometric relationship is now commonly referred to as ‘Rensch's rule’. However, despite numerous recent studies, we still do not have a general explanation for this allometry. Here we report that patterns of allometry in over 5300 bird species demonstrate that Rensch's rule is driven by a correlated evolutionary change in females to directional sexual selection on males. First, in detailed multivariate analysis, the strength of sexual selection was, by far, the strongest predictor of allometry. This was found to be the case even after controlling for numerous potential confounding factors, such as overall size, degree of ornamentation, phylogenetic history and the range and degree of size dimorphism. Second, in groups where sexual selection is stronger in females, allometry consistently goes in the opposite direction to Rensch's rule. Taken together, these results provide the first clear solution to the long-standing evolutionary problem of allometry for sexual size dimorphism: sexual selection causes size dimorphism to correlate with species size.

Allometry for Sexual Size Dimorphism: Testing Two Hypotheses for Rensch’s Rule in the Water StriderAquarius remigis

Within any given clade, male size and female size typically covary, but male size often varies more than female size. This generates a pattern of allometry for sexual size dimorphism (SSD) known as Rensch's rule. I use allometry for SSD among populations of the water strider Aquarius remigis (Hemiptera, Gerridae) to test the hypothesis that Rensch's rule evolves in response to sexual selection on male secondary sexual traits and an alternative hypothesis that it is caused by greater phenotypic plasticity of body size in males. Comparisons of three populations reared under two temperature regimes are combined with an analysis of allometry for genital and somatic components of body size among 25 field populations. Contrary to the sexual-selection hypothesis, genital length, the target of sexual selection, shows the lowest allometric slope of all the assayed traits. Instead, the results support a novel interpretation of the differential-plasticity hypothesis: that the traits most closely associated with reproductive fitness (abdomen length in females and genital length in males) are "adaptively canalized." While this hypothesis is unlikely to explain Rensch's rule among species or higher clades, it may explain widespread patterns of intraspecific variation in SSD recently documented for many insect species.

Rensch's Rule Crosses the Line: Convergent Allometry of Sexual Size Dimorphism in Hummingbirds and Flower Mites.

Hummingbirds demonstrate the full scope of Rensch's rule (females larger than males in small species, males larger than females in large species). The phoretic flower mites that hummingbirds compete with for nectar show the same pattern. An analysis of phylogenetically independent contrasts for 154 hummingbird and 37 mite species confirmed that both small (female-larger) and large (male-larger) species contribute to this result. I propose a common cause for this convergent pattern of sexual size dimorphism, with support from ecological and behavioral data for both groups. When morphology or behavior constrains each species to a limited range of resource densities, natural selection tends to diversify body size among species. If mating is promiscuous or polygynous, sexual selection favors larger males when resources are sufficient. But if male reproductive behavior is energetically costly, smaller males may have an advantage when resources are limiting, producing a pattern of allometry for sexual size dimorphism that conforms with Rensch's rule. Stabilizing selection on female size, if it occurs, amplifies this pattern. When the resource-density spectrum is broad enough, both female-larger and male-larger size dimorphism should occur within the same clade.

Rensch's Rule Crosses the Line: Convergent Allometry of Sexual Size Dimorphism in Hummingbirds and Flower Mites

Hummingbirds demonstrate the full scope of Rensch’s rule (females larger than males in small species, males larger than females in large species). The phoretic flower mites that hummingbirds compete with for nectar show the same pattern. An analysis of phylogenetically independent contrasts for 154 hummingbird and 37 mite species confirmed that both small (female‐larger) and large (male‐larger) species contribute to this result. I propose a common cause for this convergent pattern of sexual size dimorphism, with support from ecological and behavioral data for both groups. When morphology or behavior constrains each species to a limited range of resource densities, natural selection tends to diversify body size among species. If mating is promiscuous or polygynous, sexual selection favors larger males when resources are sufficient. But if male reproductive behavior is energetically costly, smaller males may have an advantage when resources are limiting, producing a pattern of allometry for sexual size dimorphism that conforms with Rensch’s rule. Stabilizing selection on female size, if it occurs, amplifies this pattern. When the resource‐density spectrum is broad enough, both female‐larger and male‐larger size dimorphism should occur within the same clade.

Allometry for Sexual Size Dimorphism: Pattern and Process in the Coevolution of Body Size in Males and Females

Sexual size dimorphism (SSD) is common in both plants and animals, and current evidence suggests that it reflects the adaptation of males and females to their different reproductive roles. When species are compared within a clade, SSD is frequently found to vary with body size. This allometry is detected as β ≠ 1, where β is the slope of a model II regression of log(male size) on log(female size). Most frequently, β exceeds 1, indicating that SSD increases with size where males are the larger sex, but decreases with size where females are larger, a trend formalized as “Rensch's rule.” Exceptions are uncommon and associated with female-biased SSD. These trends are derived from a sample of 40 independent clades of terrestrial animals, primarily vertebrates. Their extension to plants and aquatic animals awaits quantitative assessments of allometry for SSD within these groups. Many functional hypotheses have been proposed to explain the evolution of allometry for SSD, most featuring sexual selection on males or reproductive selection on females. Of these, the hypothesis that allometry evolves because of correlational selection between the sexes appears most promising as a general model but remains untested.

A Comparative Analysis of Allometry for Sexual Size Dimorphism: Assessing Rensch's Rule

Rensch's rule states that sexual size dimorphism (SSD) increases with body size (hyperallometry) in taxa in which males are the larger sex and decreases with body size (hypoallometry) in those in which females are larger. We use the independent contrasts method to assess the validity and generality of Rensch's rule within 21 independent animal taxa. Allometry is estimated as the slope of the major axis regression of contrasts for log(female size) versus contrasts for log(male size). Allometry consistent with Rensch's rule is significant in 33% of the taxa examined across a diverse range of invertebrate and vertebrate taxa. Significant allometry inconsistent with Rensch's rule occurs in only one taxon. Meta-analysis of these results reveals that Rensch's rule is general and highly significant. Only owls have allometry inconsistent with this trend. Rensch's rule is also shown to be associated with male-biased SSD, which is consistent with the hypothesis that sexual selection acting on male size drives the evolution of this pattern of allometry.

Life-history evolution in Australian snakes: a path analysis.

I recently attempted to investigate interspecific patterns in ecological traits of Australian snakes using univariate statistical techniques (Shine 1994), but high intercorrelations among variables (especially with mean adult body size) made it difficult to interpret the observed patterns. In the present paper, I attempt to tease apart causal factors using multivariate (path) analysis on the same data set (103 species, based on dissection of >22000 museum specimens). Two separate path analyses were conducted: one that treated each species as an independent unit (and thus, ignored phylogeny) and the other based on independent phylogenetic contrasts. Path coefficients from the two types of analyses were similar in magnitude, and highly correlated with each other, suggesting that most interspecific patterns among traits may reflect functional association rather than phylogenetic conservatism. Path analysis showed that indirect effects of one variable upon another (i.e., mediated via other traits) were often stronger than direct effects. Thus, even when two variables appeared to be uncorrelated in the univariate analysis, this apparent lack of relationship sometimes masked strong but conflicting indirect effects. For example, a tradeoff between clutch size and offspring size tends to mask the direct effect of mean adult body size on clutch size. Path analysis may also suggest original causal hypotheses. For example, interspecific allometry of sexual size dimorphism (as seen in Australian snakes, and many other animal groups) may result from a strong effect of another allometrically-tied trait (offspring size) on growth trajectories of females.